Field of the Invention
The present invention relates to an image forming apparatus that allows an operator to exchange fixing devices, a control method therefor, and a storage medium storing a control program therefor.
Description of the Related Art
An image forming apparatus called an MFP (Multi Function Printer) equipped with functions, such as a printer, scanner, and facsimile, is used widely in recent years. Some of such image forming apparatuses enable attachment and detachment of a plurality of kinds of fixing devices.
There are two reasons to enable attachment and detachment of a plurality of kinds of fixing devices to an image forming apparatus as follows. The first reason is to use the optimal fixing device according to a sheet type. For example, when a special sheet like an envelope is used for printing, a fixing device optimal to the special sheet is used. Since an envelope is made by folding a sheet and pasting its edges, an envelope is thicker than a normal sheet. Accordingly, when an envelope is printed with a fixing device (hereinafter referred to as a normal fixing device) that is used for a normal sheet with a general thickness, a pasted portion may deviate when passing through the fixing device, which may cause a wrinkle on the envelope.
In order to prevent such a wrinkle, a fixing device (hereinafter referred to as an envelope fixing device) that is dedicated to an envelope is prepared. Holding pressure to a sheet passing through the fixing device is set low in the envelope fixing device to prevent a wrinkle on an envelope.
The second reason is not to stop the operation of an image forming apparatus for long time due to a defect of a fixing device. In a field of a production print, it is required to print printed matter in massive pages or massive number of copies. Accordingly, when a defect occurs in a fixing device, an image forming apparatus is restored from a trouble in short time by exchanging fixing devices without repairing the defect part. Accordingly, an operator prepares a plurality of fixing devices at an operation site of an image forming apparatus, and is employing the image forming apparatus.
In this way, in the print-on-demand market in which an operator always resides, a plurality of fixing devices are prepared for an image forming apparatus, and the operator exchanges fixing devices when a defect occurs in a fixing device. In such environment, there is a known image forming apparatus that corrects difference in color tones of fixing devices (see Japanese Laid-Open Patent Publication (Kokai) No. 2013-088683 (JP 2013-088683A)). The image forming apparatus identifies the type of a fixing device, reads a gamma correction chart corresponding to the identified type of the fixing device, and applies gamma correction to image data. In the print-on-demand market in which a plurality of fixing devices are exchanged and employed, it is effective to identify a fixing device and to correct according to characteristics of the fixing device.
As mentioned above, the image forming apparatus disclosed in the publication prepares the correction data for every fixing device, and is trying to adjust the color tone corresponding to a fixing device. However, the image forming apparatus disclosed in the above-mentioned publication does not consider an effect of a relation between a fixing device and a sheet in the color tone. As mentioned above, when printing on an envelope, the image forming apparatus needs to be equipped with an envelope fixing device in order to prevent a wrinkle. However, the image forming apparatus disclosed in the above-mentioned publication cannot check whether the sheet type corresponding to a print job is the same as the sheet type allocated to a fixing device attached to the image forming apparatus. As a result, the image forming apparatus may not print suitably on some sheets.
Furthermore, some operators employ the image forming apparatus by using different fixing devices separately according to a sheet size or a sheet size series. In such a case, even when a normal fixing device is used, continuous printing on sheets of the same size may cause a scratch (edge scratch) that is the cause of output unevenness on a fixing roller.
Accordingly, when a fixing device that repeats printing on sheets of the same size fixes a toner image onto a sheet of the larger size, output unevenness caused by the repeated printing may occur in an output matter.
In order to avoid a wrinkle of an envelope and output-unevenness resulting from difference in sheet sizes at the time of printing, it is necessary to select a fixing device suitable for the type and size of a sheet defined by a print job as mentioned above. However, the image forming apparatus disclosed in the above-mentioned publication prints without considering an employment of a fixing device according to the type and size of a sheet, which reduces gross, weakens an adhesion of toner, and causes output unevenness in an output matter.